Correct Answer: D. Low heat cement
Solution:
Massive concrete structures like dams are prone to thermal cracking due to the large amount of heat generated during hydration. Low heat cement is specifically designed to have a slow rate of hydration, which significantly reduces the heat of hydration. This minimizes the temperature rise within the concrete mass, thereby preventing thermal cracks and ensuring the long-term integrity of the structure.
Correct Answer: C. Calcium oxide
Solution:
Calcium oxide (CaO), commonly known as lime, is the principal component of Ordinary Portland Cement. It typically constitutes about 60% to 67% of the total composition, making it the ingredient with the highest percentage. It is essential for the formation of the strength-giving compounds in cement.
Correct Answer: B. Hydrophobic cement
Solution:
Hydrophobic cement is manufactured by adding a small percentage of a water-repellent film-forming substance, such as oleic acid or stearic acid, during the grinding process. This substance coats the cement particles, making them resistant to hydration during storage in damp conditions, thus preserving the cement's quality for a longer period.
Correct Answer: C. Iron oxide
Solution:
The characteristic grey color of ordinary Portland cement is primarily due to the presence of iron oxide. To produce white cement, the amount of iron oxide is kept extremely low, typically below 0.5%. This is achieved by using raw materials like china clay and pure limestone which are naturally low in iron oxide.
Correct Answer: B. A retarder
Solution:
Calcium sulphate, in the form of gypsum, is added to cement during manufacturing to act as a retarder. It controls the initial setting time by slowing down the rapid hydration of Tricalcium Aluminate (C₃A), preventing a "flash set" and allowing time for the concrete to be placed and finished.
Correct Answer: B. Estimate presence of magnesia in cement.
Solution:
The Le Chatelier's apparatus is used for the soundness test, which measures the expansion of cement. This expansion is primarily caused by the presence of excess free lime (CaO). The apparatus is not sensitive to the presence of magnesia (MgO). The Autoclave test is required to determine the combined expansion due to both lime and magnesia. Therefore, the statement that it estimates the presence of magnesia is incorrect.
Correct Answer: C. Moderate rate
Solution:
The rate of cooling of cement clinker affects the formation and crystallization of the Bogue compounds. A moderate rate of cooling is optimal for developing high compressive strength. Very slow cooling can lead to large, less reactive crystals, while very fast cooling (quenching) can create a glassy, amorphous phase, neither of which is ideal for strength.
Correct Answer: A. 1:3
Solution:
The standard procedure for testing the compressive (crushing) strength of cement involves creating mortar cubes. According to IS 4031 (Part 6), these cubes are made from a mortar with a cement to standard sand proportion of 1:3 by weight.
Correct Answer: A. C₃A
Solution:
Tricalcium Aluminate (C₃A) is the most reactive of the four main compounds in Portland cement. It reacts almost instantly upon contact with water, generating significant heat and causing the initial set of the cement paste. This is why gypsum is added to control its reaction rate.
Correct Answer: C. Tricalcium aluminate
Solution:
The initial setting of cement is primarily caused by the hydration of Tricalcium Aluminate (C₃A). It is the most reactive compound and reacts rapidly with water, leading to the stiffening of the cement paste. Gypsum is added specifically to regulate this reaction and ensure a workable initial setting time.